Chemical-mechanical planarization ("CMP") processes are frequently used to planarize the surface layer of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a planarizing surface on a polishing pad is covered with a slurry solution containing small, abrasive particles and reactive chemicals. A wafer is mounted in a wafer holder, and the wafer holder is positioned opposite the polishing pad. The wafer and/or the polishing pad are then moved relative to one another allowing the abrasive particles in the slurry to mechanically remove the surface of the wafer, and the reactive chemicals in the slurry to chemically remove the surface of the wafer.
CMP processes must consistently and accurately planarize a uniform, planar surface on the wafer at a desired end-point. Many microelectronic devices are typically fabricated on a single wafer by depositing layers of various materials on the wafer, and manipulating the wafer and the other layers of material with photolithographic, etching, and doping processes. In order to manufacture ultra-high density integrated circuits, CMP processes must provide a highly planar surface so that the geometries of the component parts of the circuits may be accurately positioned across the full surface of the wafer. Integrated circuits are generally patterned on a wafer by optically or electromagnetically focusing a circuit pattern on the surface of the wafer. If the surface of the wafer is not highly planar, the circuit pattern may not be sufficiently focused in some areas, resulting in defective devices. Therefore, it is important to consistently and accurately create a uniformly planar surface on the wafer.
Several factors influence the uniformity of a planarized surface of a wafer, one of which is the distribution of the slurry between the polishing pad and the wafer. A uniform distribution of slurry between the pad and the wafer results in a more uniform surface on the wafer because the abrasive particles and the chemicals in the slurry will react more evenly across the whole wafer. One type of polishing pad provides a number of wells in the pad substrate that are uniformly spaced apart from one another across the surface of the pad. Each well holds a volume of slurry, and as the pad passes across the surface of the wafer, the slurry is drawn out of the wells into the space between the wafer and the pad. As the slurry is drawn out of the wells, a vacuum is created in the wells that holds the wafer next to the planarizing surface of the pad.
CMP processes must also provide a high throughput of finished devices to lower the unit cost of each device. The wafers, therefore, are generally between six inches and eight inches in diameter so that hundreds of microelectronic devices may be simultaneously fabricated on a single wafer. When six to eight inch diameter wafers are planarized in the presence of a slurry, however, a significant surface tension exists between the wafer, slurry, and polishing pad that holds the wafers next to the polishing pad.
One problem with current CMP planarizers is that after the CMP process is finished, it is difficult to remove large wafers from conventional polishing pads, or any wafer from polishing pads with slurry wells. Wafers are attached to the wafer carrier by drawing a vacuum on the backside of the wafer that is low enough to prevent the wafer from being damaged. After planarizing, wafers are conventionally removed from polishing pads by simply lifting the wafer carrier. Such a low vacuum, however, generally does not provide enough force to overcome the surface bond between large wafers and the polishing pads. Similarly, such low vacuums are also insufficient to overcome the bond between wafers and polishing pads with slurry wells. Therefore, it would be desirable to develop a CMP machine that can separate virtually any type of wafer from any type of polishing pad.